Hepatitis C virus (HCV) is the major etiologic agent of chronic viral hepatitis in the United States. The majority of infected individuals will develop progressive liver disease, which may evolve to cirrhosis and hepatocellular carcinoma. Treatment with interferon (IFN), plus or minus Ribavirin, is the only currently available therapy, but is not effective in the majority of patients. A molecular model for IFN-resistance has recently been proposed. In this model the NS5A protein from IFN-resistant HCV blocks the antiviral effects of IFN by inhibiting the cellular kinase PKR, a primary mediator of the cell's IFN-induced antiviral response mechanism. IFN-sensitive viruses do not block the antiviral effects of IFN because they contain mutant NS5A proteins that do not inhibit PKR. Although this model is very intriguing, the data are not sufficient to validate NS5A as a therapeutic target. The studies outlined in this proposal are aimed at validating NS5A as a therapeutic target. PROPOSED COMMERCIAL APPLICATIONS: When used in combination with interferon therapy, NS5A inhibitors have potential therapeutic use in reducing the likelihood of interferon resistance, thereby contributing to more effective treatment regimens against HCV infection.